The present invention relates to semiconductor manufacture. More particularly, the present invention relates to the in-situ stripping of photoresist from a semiconductor wafer, the wafer including at least one layer of organosilicate low-xcexa dielectric, during etching of the wafer.
Integrated circuits use dielectric layers, which have typically been formed from silicon dioxide, SiO2, to insulate conductive lines on various layers of a semiconductor structure. As semiconductor circuits become faster and more compact, operating frequencies increase and the distances between the conductive lines within the semiconductor device decrease. This introduces an increased level of coupling capacitance to the circuit, which has the drawback of slowing the operation of the semiconductor device. Therefore, it has become important to use dielectric layers that are capable of effectively insulating conductive lines against such increasing coupling capacitance levels.
In general, the coupling capacitance in an integrated circuit is directly proportional to the dielectric constant, xcexa, of the material used to form the dielectric layers. As noted above, the dielectric layers in conventional integrated circuits have traditionally been formed of SiO2, which has a dielectric constant of about 4.0. As a consequence of the increasing line densities and operating frequencies in semiconductor devices, dielectric layers formed of SiO2 may not effectively insulate the conductive lines to the extent required to avoid increased coupling capacitance levels.
In an effort to reduce the coupling capacitance levels in integrated circuits, the semiconductor industry has engaged in research to develop materials having a dielectric constant lower than that of SiO2, which materials are suitable for use in forming the dielectric layers in integrated circuits. To date, a number of promising materials, which are sometimes referred to as xe2x80x9clow-xcexc materialsxe2x80x9d, have been developed. Many of these new dielectrics are organic compounds.
LOW-xcexa materials include, but are specifically not limited to: benzocyclobutene or BCB; Flare(trademark) manufactured by Allied Signal(copyright) of Morristown, N.J., a division of Honeywell, Inc., Minneapolis, Minn.; one or more of the Parylene dimers available from Union Carbide(copyright) Corporation, Danbury CT; polytetrafluoroethylene or PTFE; and SiLK(copyright). One PTFE suitable for IC dielectric application is SPEEDFILM(trademark), available from W. L. Gore and Associates, Inc, Newark, Del. SiLK(copyright), available from the Dow(copyright) Chemical Company, Midland, Mich., is a silicon-free BCB.
One interesting class of organic low-xcexa materials are compounds including organosilicate glass, or OSG. By way of example, but not limitation, such organosilicate dielectrics include CORAL(trademark) from Novellus of San Jose, Calif.; Black Diamond(trademark) from Applied Materials of Santa Clara, Calif.; Sumika Film(copyright) available from Sumitomo Chemical America, Inc., Santa Clara, Calif., and HOSP(trademark) from Allied Signal of Morristown, N.J. Organosilicate glass materials have carbon and hydrogen atoms incorporated into the silicon dioxide lattice which lowers the dielectric constant of the material.
During semiconductor wafer processing, features of the semiconductor device are defined in the wafer using well-known patterning and etching processes. In these processes a photoresist (PR) material is deposited on the wafer and then is exposed to light filtered by a reticle. The reticle is generally a glass plate that is patterned with exemplary feature geometries that blocked light from propagating through the reticle.
After passing through the reticle, the light contacts the surface of the photoresist material. The light changes the chemical composition of the photoresist material such that a developer can remove a portion of the photoresist material. In the case of positive photoresist materials the exposed regions are removed, and in the case of negative photoresist materials the unexposed regions are removed. Thereafter the wafer is etched to remove the underlying material from the areas that are no longer protected by the photoresist material and thereby define the desired features in the wafer. Low-xcexa organic polymers in general can be etched by oxidation (e.g. oxygen-based) or reduction (e.g. hydrogen-based) chemical processes. OSG dielectrics may be advantageously etched using chemistries somewhat similar to oxide etch chemistries. OSG etch chemistries typically require etchant gasses with lower polymerization potential than straight oxide etching. This is necessary to account for the organic component in OSG films.
The etching of dielectrics may be advantageously accomplished in a dual-frequency capacitively-coupled, (DFC) dielectric etch system. One such is Lam(copyright) Research model 4520XLE(trademark), available from Lam(copyright) Research Corporation, Fremont Calif. The 4520XLE(trademark) system processes an extremely comprehensive dielectric etch portfolio in one system. Processes include contacts and vias, bilevel contacts, borderless contacts, nitride and oxide spacers, and passivation.
Advanced etch systems like the 4520XLE(trademark) perform several processes in the same system. By performing many different semiconductor fabrication steps in a single system, wafer throughput can be increased. Even further advanced systems contemplate the performance of additional steps within the same equipment. Again by way of example, but not limitation, Lam(copyright) Research Corporation""s Exelan(trademark) system is a dry etch system capable of performing many process steps in a single apparatus. Exelan(trademark) enables hardmask open, inorganic and organic ARC etch, and photoresist strip to be performed in situ with a single chamber. This system""s extensive process portfolio includes all dual damascene structures, contacts, vias, spacers, and passivation etch in doped and undoped oxides and low-xcexa dielectrics required in the sub-0.18 micron environment. Of course, the principles enumerated herein may be implemented in wide variety of semiconductor fabrication systems, and these principles specifically contemplate all such alternatives.
As used herein, the term in situ refers to one or more processes performed on a given substrate, such as a silicon wafer, in the same piece of semiconductor fabrication equipment without removing the substrate from the equipment.
During fabrication of a semiconductor device, it is necessary during the repeated patterning, etching, and deposition of the various film layers which make up the device to remove the patterned photoresist following an etching or deposition step. While a number of photoresist removal technologies and methods have been implemented, in order to maintain the high throughput required by today""s semiconductor manufacturers, the stripping of photoresist from semiconductor wafers within the etching equipment is highly desirable. As discussed, some advanced semiconductor etching devices provide the capability of performing multiple processes in situ within a single device. The normal procedure for doing such an in situ post etch photoresist strip involves using oxygen in the strip process. The use of oxygen with OSG materials is, however, problematic.
Because OSG materials are basically organically doped oxides, most current photoresist materials tend to have similar chemical characteristics with the organic component of OSG material. Accordingly, when utilizing a known oxygen-based methodology to remove organic material such as photoresist from the cap plate at the wafer""s surface, known O2 strip processes have the capability of removing organic material not only from the surface of the wafer, but potentially may also deleteriously remove organic material from the sidewall of the etched feature, or any other exposed surface. Moreover, OSG materials are susceptible to oxidation when exposed to oxygen plasma. The oxygen removes carbon and hydrogen from the OSG film, thereby rendering the films unstable and causing the dielectric constant of the film to increase.
Traditionally, etching and photoresist (PR) stripping were conducted in separate apparatus. To maximize system throughput, it would make sense to proceed from etching to PR strip in the same system. Indeed, if the two processes were performed in the same reaction vessel this would result in even greater increases in process efficiency. This leads to an interesting thought: could a system be designed in such a way that not only is etching and PR stripping conducted in the same chamber in the same system, but that these two processes be performed without de-energizing the plasma used for the etch step and utilizing it for the PR strip?
Hydrogen-based photoresist stripping has been performed in the past, typically with a barrel ash. Because of concerns for the safety of the process, this hydrogen based PR strip was typically performed at relatively low hydrogen concentrations in a nitrogen diluent, usually with hydrogen concentrations in the range of 4 to 5 percent.
What is desired is a methodology for performing dry photoresist stripping of OSG materials utilizing a hydrogen-based stripping process.
What is also desired is the methodology be practicable without the concomitant degradation in the dielectric performance of OSG materials occasioned by the use of known oxygen-based photoresist strip methodologies.
In order to maintain a high wafer throughput, what is also desirable is that the methodology be capable of being performed in situ within the fabrication equipment utilized to form the wafer.
Finally, it is desirable that the process be performed with either no or minimal post-strip residue remaining on the film surface, post-strip.
These and other features of the present invention will be described in more detail in the section entitled detailed description of the preferred embodiments and in conjunction with the following figures.
The present invention teaches a method for stripping photoresist from a semiconductor wafer including a layer of organosilicate dielectric. Following an etch step, the method introduces a flow of hydrogen-containing gas to the wafer, and uses the hydrogen-containing gas to form a plasma in proximity with at least a portion of the wafer. The plasma is used to strip at least a portion of the photoresist from the wafer. Where the stripping of the photoresist from the semiconductor wafer is performed subsequent to an etching step performed on the wafer in an etch apparatus, the present invention in turn enables the stripping of the photoresist in situ within the etch apparatus.
Prior hydrogen-based stripping efforts utilized low concentrations of hydrogen gas within a diluent gas, resulting in generally poor strip rates. Low hydrogen concentrations were evidently thought necessary for safety reasons. A surprising result of the research leading to the making of the present invention is that dramatically elevated concentrations of hydrogen in the strip or etchant gas not only enable high throughput strip rates, but the utilization of these highly concentrated hydrogen gas mixtures can be performed in safety.